It seems to me that if you find the early game fun, and the late game not-so-fun, that perhaps there are a couple of issues here:
- You've not got enough turns in the early game to fully exploit your early game units and technologies
- Micromanagement in the late game can get painful, especially when you've got lots of cities/units
In "vanilla" Civ5, I'd probably attempt to fix these issues by slowing down the game's pace, and ensuring that finding enemies to war with is relatively easy. The overall goal would be to expand quickly, and then enter the war state, and dominate long before the other victory conditions come into play.
By slowing the game pace, there will be more turns for moving units between scientific advances or production. This focuses the game more on careful unit tactics and allows for more strategic movement of units. If you're a true warmonger, you're likely to conquer the world long before you reach the top of the science tree.
Things like a small, Pangea-style map, and more civilizations than the default number would both be things that cause a great deal of war early on. Ideally, you're going for a domination victory, or maybe cultural - time, diplomacy, and science victories stretch long into the endgame and require a sizeable empire, usually.
Additionally, to reduce the amount of micromanagement I have to take on, I tend to prefer to build few cities, and conquer/puppet whenever possible. Turns pass much quicker when you've got few things to do.
Of course, since Civ5 supports mods, there are a couple of ways to attack this problem using mods:
- You could slow general science progress down, with a mod like "SloMo Science"
- You could make early science development slower with a mod like "Slower Science Start"
- You could use a larger mod that is tailored for this type of experience, such as "Wars of the Bronze Age"
You can keyword search the mod list, so you can use any part of the mod name to locate these mods and see if they tailor your experience in the way you want.
It might also be good to look for mods that change the victory conditions - I don't know what you'd consider to be "fun" as far as victory is concerned, but I'm sure mods exist that would end the game before it gets to the modern/micromanagement era if that's more your liking.